Laser-diode device including heat-conducting walls, semiconductor strips and isolating seals and process for making laser-diode device

ABSTRACT

Laser device with semiconducting diodes. The invention concerns a device with semiconducting laser diodes comprising individual wafers (8), made of a good heat-conducting material, assembled parallel to one another with semiconducting diode connector bars (9) placed between them as braces, said connector bars only taking up a longitudinal portion of the spaces between the wafers, in which they are located, and the longitudinal portions (17) of said spaces between the wafers, not taken up by said semiconducting connector bars, being used as flow channels for a cooling fluid.

BACKGROUND OF THE INVENTION

The subject of the present invention is semiconductor laser diode laserdevices and, in particular, such devices consisting of an array of laserdiodes. It also relates to a process for producing such laser devices.

A laser diode device is known, for example from U.S. Pat. No. 5,128,951,which has:

a plurality of parallel and spaced-apart walls, made of a material whichis a good heat conductor, which have at least approximately coplanarfree longitudinal edges bearing films of an electrically conductivematerial which extend on each side of said edges on both faces of saidwalls;

a plurality of semiconductor strips incorporating said laser diodes,each strip comprising an emitting surface via which the laser diodes ofthe corresponding strip emit, said semiconductor strips being housedlongitudinally in the spaces between said walls and each of saidsemiconductor strips being fastened to the opposite faces of the twowalls between which it is housed, so that said semiconductor strips areelectrically connected in series by said films of electricallyconductive material and so that the emitting surfaces of saidsemiconductor strips are at least approximately coplanar with said freelongitudinal edges of said walls; and

fluid-circulation cooling means intended for cooling said diode strips.

The known laser devices of this type include a block of material whichis a good heat conductor, which is supported by said fluid-circulationcooling means. Parallel grooves are machined, mechanically orchemically, in said block, said grooves forming the housings for saiddiode strips and defining ribs between them, each of these ribs formingone of said walls to which said diode strips are soldered. On theopposite side from the free longitudinal edges of the ribs, the latterare joined together by a base which corresponds to that part of saidblock which is not cut into by said grooves and via which said block isconnected to said cooling means.

Such known laser devices have many drawbacks.

First of all, the removal of the heat generated by said diodes is notgood. This is because, between the diode strips and the cooling fluid,the heat must travel a long path which passes through the solderedjoints between the strips and the ribs, along the height of said ribs,through said base and, finally, through the wall of said cooling meanswhich supports said base. Moreover, in order to be able to house saidstrips in said grooves easily, it is absolutely essential to provideclearances which are compensated for by the solder. Consequently, thesoldered joints between the diode strips and the grooves are thickerthan would be sufficient to ensure electrical contact, so that the rateof heat transfer is reduced at said soldered joints. This heat transferrate reduction effect is increased because of the fact that theplanarity and rugosity of the side walls of the ribs cannot be optimizedwhen cutting out said grooves. These known laser devices thereforecannot provide a high radiation density because the heat removal isinsufficient. If a high radiation density is desired, the laser diodestherefore overheat and are rapidly destroyed.

Moreover, because of the necessary existence of clearances between thediode strips and the ribs and the impossibility of applying satisfactorypressure between said strips and said ribs during soldering, thecontinuity of the electrical contact between said diode strips may bedefective, despite--or because of--the relatively large thickness ofsaid soldered joints.

Furthermore, the machining of said grooves and the fitting and solderingof said strips in them require a very high precision, not verycompatible with industrial manufacture and reasonable manufacturingcosts.

SUMMARY OF THE INVENTION

The object of the present invention is to remedy these drawbacks. Itrelates to a laser device of the abovementioned type, in which heatremoval is improved so that the homogeneity and radiation density ofsaid laser device may be high and the lifetime of the latter may belong, the structure of said laser device improved in accordance with thepresent invention making it possible, in addition, to guaranteeexcellent electrical continuity between the diode strips and to allowlow-cost industrial manufacture.

To these ends, according to the invention, the laser diode device of theabovementioned type is noteworthy:

in that said parallel and spaced-apart walls are formed by individualplates joined together, with said semiconductor strips interposedbetween them as spacers;

in that said semiconductor strips occupy only one longitudinal part ofthe spaces between plates, in which spaces they are housed; and

in that those longitudinal parts of said spaces between plates which arenot occupied by said semiconductor strips serve as circulation channelsfor said cooling fluid.

Thus, since the cooling fluid is in direct contact with said plates,there is optimum removal of the heat generated by the semiconductorstrips. Moreover, since said plates can be prepared individually, it ispossible for them to have a good planarity and a good rugosity (lessthan 20 ångstroms), so that the thickness of the soldered joints withthe strips can be minimized and so that the heat transfer at thesesoldered joints can be maximized. The diodes of said strips aretherefore effectively cooled so that they can generate high radiationdensity without overheating or being destroyed.

Preferably, in order to produce the laser diode device according to thepresent invention, the process is carried out in the following manner:

a plurality of identical rectangular individual plates made of amaterial which is a good heat conductor are prepared, at least their twolarge faces and one of their longitudinal edges are polished and saidpolished longitudinal edges and the lateral regions contiguous with saidpolished large faces are covered with films of a material which is agood electrical conductor;

a plurality of identical rectangular semiconductor diode strips, onelongitudinal edge of which serves as an emitting surface to said diodesand the two large faces of which are covered with electricallyconductive contact films, are prepared;

the contact films of said strips are applied against the lateral regionsof the films of electrically conductive material of said plates, betweenwhich are interposed electrically conductive fastening films; and

said plates and said strips are fastened together through the action ofsaid fastening films, with application of pressure.

By virtue of such a production process, especially of the application ofpressure while the diode strips are being fastened to the thermallyconductive plates, excellent electrical contact between the diodes isobtained. Moreover, this process removes all the difficulties associatedwith the aforementioned clearances and allows industrial manufacturewith relatively low manufacturing costs.

The fastening films could be made of an electrically conductiveadhesive. However, in a preferred embodiment, these fastening filmsconsist of an electrically conductive solder. Preferably, such solderfilms are in this case supported by said lateral regions of the films ofelectrically conductive material of said plates.

According to a first method of implementing the process according to theinvention:

a stack of all said plates and all said strips is produced so that:

each strip is interposed between two plates;

those longitudinal edges of said plates which are covered with a film ofelectrically conductive material are at least approximately coplanar;

the emitting surfaces of said diode strips are at least approximatelycoplanar with said longitudinal edges of said plates;

each contact film of a large face of a strip is superposed with a solderfilm of a large face of a plate; and

the entire said stack is raised to a temperature corresponding to themelting point of said solder films, while at the same time subjectingsaid stack to pressure transversely with respect to said plates andstrips, after which said stack is left to cool.

Thus, the assembly comprising said plates and said strips is produced ina single operation. However, in this case the tooling used foraccurately superposing said plates and strips and maintaining them inposition must be relatively complicated.

Thus, in order to allow simplification of this tooling, according to asecond method of implementing the process according to the presentinvention, the process is carried out in the following manner:

during the preparation of said plates, one of said lateral films ofconductive material of each plate is covered with a film of a firstelectrically conductive solder;

a plurality of subassemblies, each of which comprises a plate and astrip, are formed by superposing, each time, a plate and a strip so thatthe solder film of the plate is superposed with a contact film of thestrip and so that the longitudinal edge of said plate is at leastapproximately coplanar with the emitting surface of the strip, then byraising each subassembly to a temperature corresponding to the meltingpoint of said first solder while at the same time subjecting it topressure transversely with respect to said plate and to said strip,after which said subassembly is left to cool;

in each subassembly, the other of said lateral films of conductivematerial of the corresponding plate is covered with a film of a secondelectrically conductive solder having a melting point below that of saidfirst solder;

a stack of said subassemblies is formed by superposing them, each timeso that the other contact film of the strip of one subassembly isapplied against the film of said second solder of the plate of anothersubassembly and so that the longitudinal edges of all the plates are atleast approximately coplanar with each other and with the emittingsurfaces of said strips; and

said stack of subassemblies is raised to a temperature corresponding tothe melting point of said second solder, while at the same timesubjecting said stack to pressure transversely with respect to saidplates and strips, after which said stack of subassemblies is left tocool.

It will be noticed that this second method of implementing the processaccording to the invention makes it possible to control all saidsubassemblies before their assembly.

Preferably, the semiconductor laser diode device according to thepresent invention includes, in the spaces between plates, sealing meanswhich isolate said strips from said cooling fluid.

In a preferred embodiment, the device according to the present inventionincludes a package provided:

with a housing for said one-piece assembly comprising the individualplates and the semiconductor strips;

with means for the intake of said cooling fluid into said housing; and

with means for the discharge of said fluid out of said housing.

In this embodiment, said cooling fluid flows through the longitudinalparts of those said spaces between plates which are not occupied by saidstrips, passing through said housing of the package.

Advantageously, so as to ensure uniform flow of cooling fluid betweensaid plates, said package includes an accumulation reservoir for saidfluid, on the one hand, between said intake means and said housing and,on the other hand, between said housing and said discharge means.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures of the appended drawing will make it clearly understood howthe invention may be realized. In these figures, identical referencesdenote similar elements.

FIG. 1 is a horizontal section, along the line I--I of FIG. 2, of anillustrative embodiment of the laser diode device according to thepresent invention.

FIG. 2 is a longitudinal section of said device, along the broken lineII--II of FIG. 1.

FIG. 3 is a cross section of said device, along the line III--III ofFIG. 1.

FIG. 4 is a top view of the package of the device of FIGS. 1 to 3.

FIG. 5 is a longitudinal section of said package, along the broken lineV--V of FIG. 4.

FIG. 6 is a cross section of said package, along the line VI--VI of FIG.4.

FIG. 7 is an enlarged cross-sectional view of the one-piece assemblycomprising the individual plates and the semiconductor strips of thedevice according to the present invention.

FIGS. 8A and 8B diagrammatically illustrate the preparation of saidsemiconductor strips for the purpose of producing said one-pieceassembly.

FIGS. 8C and 8D diagrammatically illustrate the preparation of saidindividual plates for the purpose of producing said one-piece assembly.

FIG. 8E diagrammatically illustrates the fastening of said plates tosaid strips.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The laser diode device according to the present invention, and shown inFIGS. 1 to 3, includes a package 1 and a one-piece assembly 2 comprisingplates, made of a material which is a good heat conductor, andsemiconductor laser diode strips.

In the illustrative embodiment shown in these figures and in FIGS. 4 to6, the package 1 consists of several pieces, for example made of a metalsuch as aluminum or copper, joined together in a sealed manner. Thepackage 1 includes a housing 3 into which the assembly 2 may be insertedand fixed in a sealed manner, for example by adhesive bonding. Providedon each side of the housing 3 are reservoirs 4 and 5 which communicatewith the latter over its entire width. A fluid intake pipe 6 and a fluiddischarge pipe 7 run, respectively, into the reservoirs 4 and 5, saidpipes 6 and 7 being arranged diagonally with respect to each other inrelation to the housing 3.

As shown on a larger scale in FIG. 7, the assembly 2 comprises aplurality of parallel individual plates 8 joined to each other with,each time, a semiconductor strip 9 interposed between two consecutiveplates and serving as a spacer.

The individual plates 8 are made of a material having a high thermalconductivity such as, for example, beryllium oxide BeO, silicon carbideSiC, diamond, etc. This material could also be a metal, but it isadvantageous for it not to be an electrical conductor since then, aswill be seen below, water can be used as a cooling fluid. In oneparticular illustrative embodiment, the individual plates 8 were allidentical and had a rectangular parallelepipedal shape with a length of1 cm, a width of 0.3 cm and a thickness of 0.02 cm. The free (upper)longitudinal edges 10 of the individual plates 8 are coplanar, as isdiagrammatically illustrated in FIG. 7 by the line of a plane P--P. Atleast those parts of the large faces 8A and 8B of said plates 8 whichare adjacent to said longitudinal edges 10 are polished in order to bestrictly plane and have a rugosity of less than 20 ångstroms. Moreover,as illustrated in FIG. 8D, each individual plate 8 is coated, before itis incorporated into the assembly 2, with a film 11 of electricallyconductive material, including a part 11C covering its longitudinal edge10 and lateral parts 11A and 11B which partially cover, heightwise, saidlarge faces 8A and 8B. Such an electrically conductive film 11 may, forexample, be made of molybdenum or nickel, or of an alloy of such metals,and its thickness may be between 50 and 200 microns. As illustrated inFIG. 8D, the lateral parts 11A and 11B of said film 11 are respectivelycovered with a film 22A or 22B of electrically conductive solder, forexample made of an alloy of molybdenum, nickel and indium. Such a solderfilm may have a thickness of at most 10 microns.

In a known manner, the rectangular parallelepipedal semiconductor strips9 may be obtained by cutting semiconductor substrates, in which saidlaser diodes are obtained by epitaxy, into bands. The strips 9 may havea length of 1 cm, a width of 0.15 cm and a thickness of 0.02 cm.

At least the large faces 9A and 9B of the semiconductor strips 9 arepolished in order to be strictly plane and to have a rugosity of lessthan 20 angstroms. As illustrated by FIG. 8B, each semiconductor strip 9is coated, before its incorporation into the assembly 2, with anelectrically conductive contact film, for example made of gold, having apart 13A which covers the face 9A and a part 13B which covers the face9B.

In order to concentrate the laser emission through the edges 12, theopposite edges 14 of said semiconductor strips may be covered with areflective coating 15.

In order to obtain the assembly 2, an alternating stack of plates 8 andstrips 9 may be produced so that the longitudinal edges 12 of thesemiconductor strips 9, which form the emitting surfaces of the laserdiodes, are coplanar with the longitudinal edges 10 of the plates 8 (seeFIG. 7) and so that each strip 9 has its contact film 13A bearingagainst the solder film 22B of a plate 8 and its contact film 13Bbearing against the solder film 22A of another plate 8 and then pressuremay be applied transversely to said stack in order to press said solderfilms and said contact films against each other, while at the same timeheating the entire unit (for example in a hydrogen oven) to atemperature at least equal to the melting point of said solder films.After cooling, the plates 8 and the strips 9 are fastened to each otherby the solder films which, moreover, ensure that said semiconductorstrips 9 are electrically connected in series by the interaction betweensaid films 11 of electrically conductive material and said contact films13A, 13B.

As a variant, it is possible to use, for producing the solder films 22B,a metal alloy having a melting point greater than that of the metalalloy used for producing the solder films 22A. Consequently, it ispossible, in a first step, to produce subassemblies 16 (see FIG. 8E)each comprising a strip 9 and a plate 8 without the film 22A, the edges10 and 12 of which are aligned, and which are fastened by the film 22Band then, in a second step, after producing the solder film 22A on saidsubassemblies 16, to stack said subassemblies 16 in order to fasten themby their solder films 22A. Of course, both during production of thesubassemblies 16 and during their assembly, transverse pressure isapplied to said plates 8 and strips 9 while the adhesive films are beingmelted.

Whatever the embodiment used to obtain the assembly 2, it is observedthat the latter includes, between each pair of consecutive plates 8, afree longitudinal channel 17 not occupied by the correspondingsemiconductor strip 9.

As shown in FIG. 7, each longitudinal channel 17 is isolated from thecorresponding semiconductor strip by a seal 18, for example created byinjecting a silicone or an epoxy resin. Optionally, a stiffening plate19 is fixed to the longitudinal edges of the plates 8, opposite thelongitudinal edges 10.

After producing said assembly 2, the latter is placed in and fixed in aleaktight manner (for example, by adhesive bonding) to the housing 3 ofthe package 1 so that its longitudinal channels 17 communicate, on oneside, with the reservoir 14, and on the other, with the reservoir 5.

Thus, when a cooling fluid, for example water, is made to flow throughthe package 1 between the intake pipe 6 and the discharge pipe 7, saidfluid flows in parallel along said longitudinal channels 17 andeffectively removes the heat generated by the laser diodes of thesemiconductor strips 9 (see the arrows in FIGS. 1 and 2).

Electrodes 20 and 21 are electrically connected to the metal films 11 ofthe end plates of the assembly 2 and allow the diode strips to beelectrically connected in series to the terminals of a DC or pulsedpower supply. In one embodiment of the device according to theinvention, current pulses having an intensity of 100 amps and durationsof about 200 to 400 microseconds were used.

Thus, it may be seen that it is possible, with the device of the presentinvention, to obtain:

a high density and good homogeneity of the laser radiation, because ofthe compactness of the assembly of the laser diode strips 9;

a long lifetime for said device, because of the effective removal ofheat from the active region of the laser diodes; and

automation of the manufacture of said device, making it possible toreduce the manufacturing costs thereof.

The applications of the laser diode device of the present invention arenumerous and relate, for example, to the pumping of solid-state lasers,to fiber-optic links, to the laser treatment of materials, to medicine,etc.

What is claimed is:
 1. A laser-diode device, comprising:a plurality ofparallel and spaced-apart walls (8), made of a material which is a goodheat conductor, which have at least approximately coplanar freelongitudinal edges (10) bearing films (11) of an electrically conductivematerial which extend on each side of said edges on a pair of faces (8A,8B) of said walls; a plurality of semiconductor strips (9) incorporatinglaser diodes, each strip comprising an emitting surface (12) via whichsaid laser diodes of a corresponding strip emit, said semiconductorstrips (9) being housed longitudinally in spaces between said walls (8)and each of said semiconductor strips (9) being fastened to oppositefaces of two of said walls (8) between which each said strip is housed,so that said semiconductor strips (9) are electrically connected inseries by films of electrically conductive material and so that saidemitting surfaces (12) of said semiconductor strips (9) are at leastapproximately coplanar with said free longitudinal edges (10) of saidwalls (8); and fluid-circulation cooling means that cool said strips(9), wherein said parallel and spaced-apart walls are formed byindividual plates (8) joined together, with said semiconductor strips(9) interposed between said plates as spacers, wherein saidsemiconductor strips (9) occupy only one longitudinal part of saidspaces between said plates (8), in which spaces said strips (9) arehoused, wherein said longitudinal parts of said spaces between saidplates (8) which are not occupied by said semiconductor strips (9) serveas circulation channels (17) for cooling fluid, and wherein saidlaser-diode device additionally comprises sealing means (18) disposed insaid spaces between said plates (8) that separate said longitudinalparts occupied by said strips (9) from said longitudinal parts notoccupied by said strips (9).
 2. A laser-diode device according to claim1 including a package (1) comprising:a housing (3) for a one-pieceassembly (2) comprising said plates (8) and said semiconductor strips(9); means (6) for intake of said cooling fluid into said housing (3);and means (7) for discharge of said cooling fluid out of said housing(3), said cooling fluid passing through said housing (3) by flowingthrough said longitudinal parts of said spaces between said plates (8)which are not occupied by said strips (9).
 3. A laser-diode deviceaccording to claim 2, wherein said package (1) additionally comprises acooling-fluid accumulation reservoir (4 and 5) between said intake means(6) and said housing (3) and between said housing (3) and said dischargemeans (7).
 4. A process for producing the laser diode device specifiedby claim 1, comprising the steps of:preparing a plurality of identicalrectangular individual plates (8) made of a material which is a goodheat conductor, at least two faces (8A, 8B) and one of said longitudinaledges (10) of said plates (8) being polished and said polishedlongitudinal edges (10) and lateral regions contiguous with saidpolished large faces (8A, 8B) being covered with films (11A, 11B, 11C)of a material which is a good electrical conductor; preparing aplurality of identical rectangular semiconductor diode strips (9), onelongitudinal edge of each of said strips (9) serving as an emittingsurface to said diodes and said two faces of which are covered withelectrically conductive contact films (13A, 13B); applying said contactfilms of said strips against said lateral regions of said films (11A,11B) of conductive material of said plates (8), between which areinterposed electrically conductive fastening films (22A, 22B); fasteningsaid plates (8) and said strips (9) together via said fastening filmswith application of pressure; and forming said sealing means (18).
 5. Aprocess according to claim 4, wherein said lateral regions are coveredwith electrically conductive solder films.
 6. A process according toclaim 5, additionally comprising the steps of:producing a stack of saidplates (8) and said strips (9) so that each strip (9) is interposedbetween two of said plates (8), so that said longitudinal edges (10) ofsaid plates that are covered with a film (11C) of electricallyconductive material are at least approximately coplanar, so that saidemitting surfaces (12) of said diode strips (9) are at leastapproximately coplanar with said longitudinal edges (10) of said plates,and so that each of said contact films (13A, 13B) of a face of one ofsaid strips (9) is superposed with a solder film of a face of one ofsaid plates (8); and raising said stack to a temperature correspondingto the melting point of said solder films, while subjecting said stackto pressure transversely with respect to said plates and strips, afterwhich said stack is left to cool.
 7. A process according to claim 5,additionally comprising the steps of:covering one (11B) of said lateralfilms of conductive material of each plate (8) with a film (22B) of afirst electrically conductive solder during preparation of said plates(8); forming a plurality of subassemblies (16), each of which comprisesa plate (8) and a strip (9), by superposing each time, a plate and astrip so that said solder film of said plate is superposed with acontact film of said strip and so that the longitudinal edge (10) ofsaid plate is at least approximately coplanar with said emitting surface(12) of said strip, then by raising each subassembly (16) to atemperature corresponding to the melting point of said first solderwhile at the same time subjecting said subassembly to pressuretransversely with respect to said plate and to said strip, after whichsaid subassembly is left to cool; in each subassembly (16), coveringanother (11A) of said lateral films of conductive material of acorresponding plate (8) with a film (22A) of a second electricallyconductive solder having a melting point below that of said firstsolder; forming a stack of said subassemblies (16) by superposing them,each time so that on of said contact films (13B) of said strip of onesubassembly is applied against said film (22A) of said second solder ofsaid plate of another subassembly (16) and so that said longitudinaledges of said plates are at least approximately coplanar with each otherand with said emitting surfaces of said strips; and raising said stackof subassemblies (16) to a temperature corresponding to the meltingpoint of said second solder, while at the same time subjecting saidstack to pressure transversely with respect to said plates and strips,after which said stack of subassemblies is left to cool.